1. Field of the Invention
This invention relates to a soluble, and processable doped electrically conductive polymer and a polymer blend thereof, more particularly to a doped polymer and polymer blend thereof which are both doped with a protonic acid having a long carbon chain or a larger substituent group.
2. Description of the Prior Art
Conjugated conductive polymers have been widely investigated due to growing interest in their use in anti-static coatings, conductive paints, electromagnetic shielding, electrode coatings and the like. There have been many attempts to dope the conjugated conductive polymers with acids. The dopant often used in a general conjugated heterocyclic conductive polymer is a Lewis acid, such as FeCl.sub.3, SbF.sub.5, AsF.sub.5, NOPF.sub.6 or SnCl.sub.4. An oxidation reaction is employed during such a doping process. The polymer loses electrons and a delocalized polaron and/or bipolaron are formed on the conjugated chain thereof. This causes the polymer to have an increased electrical. conductivity.
In contrast to the general conjugated heterocyclic conductive polymers, the dopant often used in a polyaniline is a protonic acid. No oxidation reaction occurs, but the imine group transforms to imine salt. Since the positive charge on the N atom of the imine salt can delocalize to the benzene ring, therefore the acid-doped polyaniline has an increased conductivity. The protonic acid often used is H.sub.2 SO.sub.4, HCl, HF or HBF.sub.4 as disclosed in Macromolecules, 22, 649(1989) and Synth. Met., 24, 255(1988).
The doping will make the main chain of the polymer more rigid, resulting in an aggregation of the polymer chains and thus the polymer precipitates out from the solution. This; limits the applications of the doped conductive polymers. There are two ways to improve the drawbacks as mentioned above. One way is by a chemical method. It is disclosed that a neutral conductive polymeric film is synthesized by a chemical method, then is dipped into a solution containing an oxidizing agent for doping. The dopants used include FeCl.sub.3, Fe(ClO.sub.4).sub.3 [Synth. Met., 41-43, 825(1991)], NOPF.sub.6 [Synth. Met., 22, 103(1987)] and hydrochloric acid solution [Mol. Cryst. Liq. Cryst., 125, 309(1985)].
Another way is by an electrochemical method. It is disclosed that a conductive polymer film is synthesized by an electrochemical method. The conductive polymer film is synthesized and the anion of the supporting electrolyte (Bu).sub.4 NClO.sub.4 is also doped to the film at the same time [Synth. Met., 9, 381(1984)]. However, regardless of whether doping is achieved by the chemical or electrochemical method, the resultant film is brittle and insoluble; and a large area film is difficult to obtain. Thus its application is limited.
Incorporating a conductive polymer with a non-conductive conventional polymer to form a conductive polymer composite film has been disclosed. This can be achieved by an electrochemical or chemical method. Paoli et al. discloses that a polypyrrole/poly(vinyl chloride)(PVC) conductive composite film can be prepared by immersing an anode coated with a PVC film in a solution containing appropriate solvent, pyrrole monomer and an electrolyte. The PVC film will swell in the solution thus enabling the pyrrole to infiltrate into the PVC film to form the composite film [J. Polym. Sci., Polym. Chem. Ed., 23, 1687(1985)]. Also, a polyaniline/poly(vinyl alcohol) composite film has been prepared in our laboratory [Macromolecules, 24, 1242(1991)].
A conductive polymer composite film can also be prepared by a chemical method. Brocchi et al. discloses that a polypyrrole/polypropylene composite film is prepared by placing a polypropylene film at the interface of a 30% of ferric chloride solution and a 10% of pyrrole solution. The two solutions will diffuse into the polypropylene allowing polypyrrole to form in the bulk to give a composite film [J. Chem. Soc., Chem. Commun., 148(1986)]. Ojio et al. discloses that a polypyrrole/poly(vinyl alcohol) composite film can be prepared by exposing a poly(vinyl alcohol) film containing ferric chloride to pyrrole monomer vapor [Polymer J., 189(1), 95(1986)]. Laakso et al. discloses that a doped composite film can be prepared by blending a poly-3-octylthiophene with polyethylene, polystyrene and ethylene/vinyl acetate copolymer in the melted state, then processing the polymer blend to form a film and immersing the film in a FeCl.sub.3 /CH.sub.3 NO.sub.2 solution or exposing the film under iodine vapor for doping [Synth. Met., 28, C467(1989)].
All the above mentioned polymer composite films have a common drawback in that the monomers cannot be evenly distributed in the matrix film. Until now, a doped polyheterocyclic compound which is soluble and processable has not been disclosed. Also, a protonic acid having a long carbon chain or a large substituent group doped therein has not been disclosed either.